Method for maintaining a vapor blanket in a condensation heating facility

ABSTRACT

This disclosure is directed to maintaining a blanket of secondary vapor intermediate a hot primary vapor and the atmosphere, in a condensation heating facility, to prevent losses of the primary vapor to the atmosphere. A secondary liquid is transported through the secondary vapor into the hot primary vapors where the secondary liquid is vaporized to supplement the secondary vapor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a condensation heat transfer facility havingtherein a primary body of hot saturated vapor with a secondary body ofvapor interposed between the primary body of vapor and the atmosphere.In particular, the invention is directed to a method for maintaining thesecondary body of vapor.

2. Description of the Prior Art

Various diverse operations such as soldering, fusing, brazing, curing,cooking, etc. require that articles be heated to elevated temperatures.One method of heating articles to the desired elevated temperatures isdescribed in U.S. Pat. No. 3,904,102 to Chu et al. which issued on Sept.9, 1975 and is assigned to the assignee of the instant invention.

The Chu et al. patent describes a condensation heating facility which isopen to the atmosphere in order to facilitate the entry and removal ofarticles therefrom. The articles to be heated to elevated temperaturesare placed in a primary body of hot saturated vapors where a portion ofthe vapor will condense thereon and transfer thereto its latent heat ofvaporization to heat said articles. A secondary body of vapor of arelatively inexpensive material is interposed between the relativelyexpensive primary body of saturated vapor and the atmosphere to reduceor substantially eliminate losses to the atmosphere of the hot primaryvapor due to convection, diffusion and drag-out.

Although the secondary body of vapor has been found to be effective toreduce significantly the losses of the expensive primary vapor, aportion of both the primary and secondary vapors are continuously lostto the atmosphere across the secondary vapor-air interface. Asignificant contributor to such losses has been found to be the methodof providing supplemental secondary vapor in order to maintain thesecondary body of vapor. A most important parameter relating to theabove-referred to losses is the disturbance to the primary-secondaryvapor interface when generating supplemental secondary vapor. Such adisturbance will increase the loss of the secondary vapors and, moreimportantly will result in additional losses of the expensive primaryvapors to the atmosphere.

The secondary body of vapor may be supplemented by (a) vaporizing asecondary liquid by feeding said liquid directly into the hot primaryliquid, (b) vaporizing the secondary liquid by spraying it over the hotprimary-secondary vapor interface, or (c) providing a secondary vaporgenerator, outside the facility, which communicates with and supplementsthe body of secondary vapor.

Although all of the above methods have each met with a measure ofsuccess in maintaining the secondary body of vapor, each has definitedrawbacks. The feeding of a secondary liquid directly into the hotprimary liquid or spraying of such liquid over the primary-secondaryvapor interface causes violent mixing resulting in additional losses ofthe expensive primary vapor to the atmosphere. The use of a secondaryvapor generator necessitates additional expensive apparatus whichrequires a high degree of control.

SUMMARY OF THE INVENTION

The foregoing problems have been overcome by the instant method formaintaining a secondary body of vapor, above a primary body of vapor, ina condensation heating facility wherein a wick is mounted within thefacility. The wick transports a secondary liquid through the body ofsecondary vapor and into contact with the primary vapor to vaporize thesecondary liquid to provide supplemental vapor in order to maintain thesecondary body of vapor.

By wicking the secondary liquid through the secondary vapor zone, thesecondary liquid advantageously vaporizes slowly and uniformly over alarge surface area and violent mixing does not take place.

Advantageously, such a wicking arrangement is both inexpensive and easyto install.

Additionally, the wick can be easily removed for replacement or periodiccleaning.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying FIGURE represents diagrammatically a partial sectionalview in elevation of a condensation heat transfer facility incorporatingthe instant inventive concepts.

DETAILED DESCRIPTION

A preferred embodiment of the instant condensation heat transferfacility, generally indicated by the numeral 10, is shown in the soleFIGURE of the drawing. The facility 10 is comprised of a vessel 11, opento the atmosphere, and having a heating coil 12 which may be, forexample, electrically operated. The heating coil 12 may be supplementedby an external source of heat applied to the vessel 11 such as a hotplate or the like (not shown).

A set of primary cooling coils 13 are located intermediate the top andbottom of the vessel 11 and receive a circulating cooling medium from asource, not shown. A set of secondary cooling coils 14 are locatedadjacent the upper portion of the vessel 11 and also receive acirculating cooling medium from a source not shown. The set of secondarycooling coils 14 is operated at a lower temperature than the set ofprimary cooling coils 13.

Fixedly mounted, around the inner periphery of the vessel 11,immediately below the set of primary cooling coils 13 is a primarytrough 15 which receives primary condensate draining off the set ofprimary cooling coils 13. The primary condensate is discharged into avalved line 16 which communicates with the lower portion of the vessel11. Additionally, a secondary trough 17 is mounted around the innerperiphery of the vessel 11 immediately below the set of secondarycooling coils 14. The secondary trough 17 receives condensate drainingoff the set of secondary cooling coils 14 and discharges that condensateinto the line 18 and into a storage tank 19. The storage tank 19 mayadvantageously contain a filter to remove dirt, flux or othercontaminants from the condensate. Additionally, tank 19 is connected toa supplemental supply (not shown) of secondary liquid to provide make-upfor the secondary vapors lost to the atmosphere.

A wick 26 is suspended from a hanger 27 about the inside periphery ofthe vessel 11 and extends from a point just below the set of secondarycooling coils into the primary condensate in the trough 15. A meteredpump line 28 extends from the storage tank 19, into the upper portion ofthe vessel 11, and terminates in a plurality of spray nozzles 29--29.

In operation, a mixture of two liquids as hereinafter described, isintroduced into the vessel 11 to a level generally indicated by thenumeral 31 and is brought to and maintained at a boil by means of theheating coil 12.

One of the two liquids is a primary heat transfer liquid characterizedby the following general properties:

a. A boiling point at atmospheric pressure at least equal to, andpreferably slightly above, the temperature required for the operation tobe performed (i.e., soldering, fusing, brazing, curing, cooking etc.).For example, in a soldering operation, this boiling point is at leastequal to, and preferably above, the melting point of the solder used inthe operation.

b. Must produce a saturated vapor which is denser than air atatmospheric pressure.

c. Desirably has a well defined and substantially constant boiling pointfor better control over the process.

d. Desirably produces a saturated vapor which is non-oxidizing,chemically stable and inert, non-toxic and non-inflammable.

In addition to the general properties hereinabove recited, when thefacility is used to heat an article such as a printed circuit boardhaving mounted thereon for soldering various electrical components, theprimary liquid advantageously should not be electrically conducting.

The other of the two liquids is a secondary liquid characterized by thefollowing properties:

a. A lower boiling point at atmospheric pressure than the primaryliquid.

b. Produces a vapor which, for the embodiments disclosed herein is, atatmospheric pressure, less dense than saturated vapor from the primaryliquid at this pressure and which is denser than air at this pressureand at the same temperature.

c. Does not form an azeotrope with the primary liquid.

d. Produces a saturated vapor which does not support a high equilibriummoisture content.

e. Produces a saturated vapor which is non-oxidizing, chemically stable,non-toxic and non-inflammable.

In addition to the general properties hereinabove recited, when thefacility is used to heat an article such as a printed circuit boardhaving mounted thereon for soldering various electrical components, thesecondary liquid should not be electrically conducting.

When the mixture of the primary and secondary liquids has been broughtto a boil, the lower-boiling, non-azeotrope forming, secondary liquidwill boil off first to form a body of secondary vapors which will fillthe vessel 11 up to some level, depending upon the quantity of secondaryliquid in the liquid mixture. After the secondary liquid has boiled off,the remaining primary liquid will be further heated, by means of theheating coil 12, until its boiling point is reached, whereupon theprimary liquid will boil and produce a body of hot saturated primaryvapors. The body of primary vapors being denser than the body ofsecondary vapors, as the primary vapors are formed and rise within thevessel 11, the lighter secondary vapors will be pushed upwardly in thevessel by the said primary vapors. In effect, the secondary body ofvapors will be stably stratified over and float on the body of hotprimary vapors and provide a vapor blanket between the hot primaryvapors and the atmosphere.

The proportions of primary and secondary liquids in the mixture thereofwill be selected and determined by the geometry of vessel 11, thelocations of the sets of primary and secondary cooling coils 13 and 14,respectively, in the said vessel and the desired thickness of the layerof secondary vapor over the primary vapor. Specifically, the proportionsof primary and secondary liquids in the mixture thereof are chosen sothat, at equilibrium, after start-up as hereinabove described, therewill exist in the vessel 11 a body of hot saturated essentially primaryvapor, the top of which will be approximately at the level indicateddiagrammatically by phantom line 32 extending through the primary set ofcooling coils 13. There will also exist a body of a mixture of primaryand secondary vapors extending from the level indicated diagrammaticallyby the phantom line 32 up to a level indicated diagrammatically by asecond phantom line 33; there will also exist a body of saturated,essentially secondary, vapor extending from the level indicateddiagrammatically by phantom line 33 up to a level indicateddiagrammatically by a third phantom line 34 shown extending through theset of secondary cooling coils 14.

The set of primary cooling coils 13 is maintained at a temperature belowthe condensation temperature of the primary vapor (i.e., below theboiling point of the primary liquid) and above the boiling point of thesecondary vapor. The set of secondary cooling coils 14 is maintained ata temperature below the condensation temperature of the secondary vapor(i.e., below the boiling point of the secondary liquid).

Condensate draining off the surface of the set of primary cooling coils13, essentially primary liquid, is collected in the primary trough 15and is returned to the lower portion of the vessel 11, above the liquidlevel 31, through the valved line 16.

Condensate draining off the surface of the set of secondary coolingcoils 14, essentially secondary liquid, is collected in the secondarytrough 17 and is returned through the line 18 to the storage tank 19 andpumped through the line 28 and onto the wick 26 in the form of a finespray through the nozzles 29--29.

Prior condensation heating facilities used various means and methods tosupplement and maintain the body of secondary vapor. An offset vaporgenerator, outside the facility, communicating with the body ofsecondary vapor provided sufficient vapor to maintain the secondary bodyof vapor. However, such a generator is expensive and requires a highdegree of control. Other methods either fed the secondary liquiddirectly into the hot primary liquid or sprayed the secondary liquidonto the primary-secondary interface. Such methods caused a violentmixing which caused undesirable losses of the expensive primary vapor.

The wick 26 precludes these problems by transporting the secondaryliquid down through the body of secondary vapor and into the body of hotprimary vapors. The secondary liquid is dispersed over the relativelylarge surface area of the wick 26 resulting in a gentle and uniform flowand vaporization thereof. The body of hot primary vapor is at asubstantially higher temperature than the boiling temperature of thesecondary liquid which causes the secondary liquid to vaporize in orderto supplement and maintain the body of secondary vapor. Optimally,substantially all of the secondary liquid sprayed onto the wick willvaporize when above the condensate in the primary trough 15 and belowthe top of the body of hot primary vapors indicated by the first phantomline 32 where the temperatures are well above the boiling point of thesecondary liquid.

The wick 26 used in an exemplary embodiment was a knitted wire 50density mesh, which was 6 inches wide, with 12 strands of 0.0045 inch,316 stainless steel wire. The wick 26 is not limited to stainless steel,for other metals may be used as well as cloth fabric or othernon-deteriorating or slow deteriorating material.

Advantageously, the wick 26 may be removably supported from the hanger27 by hooks, clips, snaps or the like. Such an arrangement facilitatesthe removal of the wick 26 for cleaning, repair or replacement.

The purpose of the spray nozzles 29--29 is to deposit the secondaryliquid over a larger surface area of the upper portion of the wick 26 tofurther ensure a gentle uniform flow thereof down the wick. However, thesame result can be accomplished by feeding the secondary fluid directlyonto the upper end of the wick 26 at more closely spaced intervals. Thespacing of such intervals would depend upon the wicking ability of thewick 26 and the deposition rate of the secondary fluid onto the wick.

Care should be taken to control the deposition rate of secondary liquidsprayed onto the wick 26 for a very high injection rate could cause someof the secondary fluid to enter into the primary condensate in thetrough 15. This could cause violent boiling and turbulence and disruptthe primary-secondary interface resulting in loss of the expensiveprimary vapors.

By extending the wick 26 into the hot primary condensate in the trough15, the wick is advantageously heated which aids in the evaporation ofthe secondary liquid moving down the wick. However, it is not necessarythat the wick 26 extend into the hot primary condensate, as long as itdoes extend into the body of hot primary vapors below the phantom line32. If the secondary liquid injection rate is set properly, it willvaporize prior to reaching the lower end of the wick 26 as hereinbeforeindicated.

Additionally, the wick 26 does not have to be mounted completely aroundthe inside periphery of the vessel 11, it may cover a smaller portion ofthe inside periphery depending on the desired injection rate of thesecondary liquid into the system. A slow injection rate would require asmaller wick surface area for transporting the secondary liquid to thebody of hot primary vapor. Furthermore, the wick 26 does not have to bemounted around the inside periphery, but may be located at any positionwithin the vessel 11 as long as it does not interfere with the movementof articles into and out of the vessel.

Advantageously, the wick 26 disperses the secondary liquid moreuniformly than other methods hereinbefore discussed. The instant wickingmethod provides a gentle and uniform feeding of the secondary liquidinto the body of hot primary vapor resulting in a slow vaporization anda gentle mixing of the primary-secondary vapors. Such a gentle mixingcauses a minimal disturbance of the primary-secondary interface andsubstantially decreases the loss of the expensive primary vapors.

The operation of an exemplary embodiment shown in the FIGURE will now bedescribed specifically in connection with a soldering, although theinstant invention is not so limited. After the bodies of primary andsecondary vapors have been established in the vessel 11, as hereinabovedescribed, an article 41, which may for example, be a printed circuitboard with electrical components mounted thereon for soldering and withsolder preforms or plating as required provided thereon in a knownmanner, is lower in the vessel. The article 41 passes through the bodyof secondary vapor into the body of hot saturated primary vapor, belowthe set of primary cooling coils 13 and is held suspended in thisposition as shown in the FIGURE. Hot saturated primary vapor willcondense on the article 41, giving up latent heat of vaporization toheat the article until the temperature of the article approaches orreaches the temperature of the saturated body of hot primary vapor. Thattemperature is the boiling point of the primary heat transfer liquid,and which, as hereinbefore mentioned, is at least equal to, andpreferably above, the melting point of the solder used for theoperation. The article 41 will approach or reach the temperature of thebody of hot saturated primary vapor rapidly, because heat transfercoefficients for condensation processes are among the highest known forany mode of heat transfer. The solder will melt or reflow to effect thesoldering operation on the article (e.g., to effect the soldering ofelectrical components to a printed circuit board).

The high rate of heat transfer from the condensing hot saturated primaryvapor to the article 41, permitting the soldering operation to becompleted rapidly, results in an exposure of relatively short durationof the article to the elevated soldering temperature. Where the article41 is a printed circuit board having electrical components mounted forsoldering thereon, this relatively short exposure to the elevatedsoldering temperatures prevents heat damage to the said printed circuitboard and associated electrical components which might otherwise occurunder more prolonged exposure to such elevated temperature.

With adequate vapor generating capacity in the condensation heattransfer facility 10 (and it will be understood that heating coil 12,and auxiliary heating means such as a hot plate, if employed, are sosized as to be capable of maintaining the level of hot saturatedessentially primary vapor up to approximately the level of the firstphantom line 32 when the article 41 is introduced into such hotsaturated essentially primary vapor), soldering times may range betweenapproximately 5 seconds for typical light printed circuit boards andcomponents with low thermal mass to approximately 90 seconds for typicalmassive printed circuit boards and components with high thermal mass.Primary vapors condensing on the article 41 will drain back to the bodyof liquid in the lower portion of vessel 11. It will be seen that theprimary heat transfer liquid is continuously being distilled, thatnon-volatile contaminants remain in the body of the liquid at the bottomof the vessel 11, and that primary vapors condensing on the article 41are relatively clean. The hot saturated primary vapor, beingnon-oxidizing, may eliminate in some instances any need for fluxing thearticle 41. After the solder has melted or reflowed, the article 41 isremoved, through the body of secondary vapor blanketing the body ofprimary vapor, from the vessel 11 and cooled to ambient temperature.

During the soldering operation secondary vapor is continuouslycondensing on the cooling coils 34 (and some secondary vapor is beingcontinuously lost to the atmosphere). The secondary vapor condensate isrecirculated through the line 18 and the tank 19 (where additionalsecondary liquid may be supplied from an outside source) and is sprayedonto the wick 26 where it is wicked down to the body of hot primaryvapor. The secondary fluid is then evaporated by the hot primary vaporsto supplement and maintain the body of secondary vapors.

Although the operation hereinabove described for the FIGURE has beenbased upon soldering one article 41, it will be understood that a batchof articles may be introduced at one time into the vessel 11 forsimultaneous soldering.

In a specific example wherein the article 41 is a printed circuit board,and electrical components are to be soldered thereto with solder meltingat 360° F. (182.2° C.), the primary heat transfer liquid advantageouslyis selected from the group of liquids known generically asfluorocarbons. One such liquid is sold by E. I. DuPont de Nemours andCo. under the tradename "FREON E5" and has the following significantproperties:

Boiling point at atmospheric pressure -- 435.6° F. (224.2° C.)

Electrical resistivity -- greater than 4 × 10¹⁴ ohm-cm.

Dielectric constant -- 2.45

Latent heat of vaporization -- 19.9 BTU/No.

Density of saturated vapor at boiling point and atmospheric pressure --1.45 No./ft.³

Chemical stability, inertness, non-toxicity, non-flammability.

Another suitable primary heat transfer liquid is sold by MinnesotaMining and Manufacturing Co. under the tradename "FLUORINERT FC-70" andhas the following significant properties:

Boiling point at atmospheric pressure -- 419° F. (215° C.)

Dielectric constant -- 1.94

Latent heat of vaporization -- 23 BTU/No.

Density of saturated vapor at boiling point and atmospheric pressure --1.27 No./ft.³

Chemical stability, inertness, non-toxicity, non-flammability.

A suitable secondary liquid adapted for use in the manner hereinbeforedescribed with either "FREON E5" or "FLUORINERT FC-70" as the primaryheat transfer liquid advantageously is selected from the group ofliquids known generically as halogenated hydrocarbons such astrichloro-trifluoro-ethane. Such a liquid is sold by E. I. DuPont deNemours and Co. under the tradename "FREON TF" and has the followingsignificant properties:

Boiling point at atmospheric pressure -- 117.6° F. (47.6° C.)

Electrical resistivity -- greater than 2 × 10¹⁵ ohm-cm.

Dielectric constant -- 2.41

Latent heat of vaporization -- 63.12 BTU/No.

Density of saturated vapor at boiling point and atmospheric pressure --0.461 No./ft.³

Substantial chemical stability, very low toxicity, non-flammability.

The facility 10 shown in the FIGURE was successfully operated, employed"FLUORINERT FC-70" as the primary liquid and "FREON TF" as the secondaryliquid, "FREON TF" constituting 5 percent by volume of the mixture ofthe two liquids. The primary set of cooling coils was operated atapproximately 125° F. (51.7° C.) and the secondary set of cooling coilsat approximately 40° F. (4.4° C.).

What is claimed is:
 1. A method for maintaining a body of secondary vapor in a condensation heating facility having a vessel, open to the atmosphere, containing a body of hot primary vapor with the body of secondary vapor interposed between the body of primary vapor and the atmosphere, the method comprising the steps of:depositing a secondary liquid on a wick mounted within the vessel, and wicking the secondary liquid through the secondary body of vapor and into contact with the body of hot primary vapor to vaporize the secondary liquid to provide supplemental secondary vapor to maintain the body of secondary vapor.
 2. The method of claim 1 wherein the depositing step is accomplished by:spraying the secondary liquid onto the wick. 